Process points of heat treatment of large diameter springs with high frequency hardening machine
Large-diameter springs are made of hot coils. As springs for large valves, they have to withstand repeated elongation and compression during operation. Therefore, they should have excellent elasticity and fatigue strength. The failure modes of the spring are mainly fatigue fracture and Stress relaxation, and about 90% of springs fail due to fatigue fracture. According to its service conditions, 50CrVA spring steel with good hardenability, small deformation and good mechanical properties must be selected. After quenching + medium temperature tempering by high-frequency hardening machine, it can fully meet its work needs. Today, I will tell you about its high-frequency heat treatment process.
(1) Heat treatment process
a. The spring before rolling is made of abrasive materials, and the heating of the spring is carried out by a high-frequency hardening machine. It has the characteristics of short heating time and fine austenite grains. Due to the fine austenite grains, the material body is increased. The number of structure grains and the area of grain boundaries reduce stress concentration and increase the resistance of dislocation movement. The heating temperature is (900±10)℃. At this time, the material’s high temperature strength and good plasticity are used to make the rolling easy. However, the heating temperature should not be too high or the holding time is too long, otherwise the material will overheat or surface Oxidation and decarburization may even result in overburning and scrapping.
b. Quenching + tempering at medium temperature. Heating is carried out on the high-frequency hardening machine, the heating temperature is 850-880℃, the heat preservation coefficient is calculated at 1.5min/mm, based on the through firing, the cooling medium has an important influence on the hardness and performance of the spring, and oil cooling can be selected. Meet its process requirements.
c. Tempering is also carried out by high-frequency quenching machine. According to the requirements of hardness, perpendicularity and gap, use special tempering fixture to fix and place it correctly. The heating temperature is 400-440℃, and the water is cooled after heat preservation. The tempering temperature of general springs is generally 400-500℃, and higher fatigue strength can be obtained after tempering.
(2) Analysis and implementation points of heat treatment process of spring
① Because 50CrVA steel has many alloying elements, the hardenability of the steel is improved. Chromium is a strong carbide element, and their carbides exist near the grain boundary, so it can effectively prevent the growth of grains, so it is appropriately improved Quenching temperature and prolonging the holding time will not cause the growth of crystal grains.
②In the heating process of hot coil springs, attention should be paid to the relationship between surface decarburization and quenching heating temperature and time. Practice has shown that high quenching temperature and long heating time will cause decarburization to increase. Therefore, when high-frequency quenching machine is used for heating, the process parameters should be strictly controlled. In addition, coating or packing protection heating can also be used to reduce the oxidation and decarburization of the surface. There are literatures that the surface decarburization of the spring reduces its service life, and it is easy to become a source of fatigue cracks.
③Medium temperature tempering of the spring is to obtain the required microstructure and performance. Considering that 50CrVA steel is a material that produces second temper brittleness, it must be cooled quickly (oil or water cooling) after tempering to prevent temper brittleness (causing Its impact toughness is reduced), and it can cause residual compressive stress on the surface, which is beneficial to improve the fatigue strength. Usually, water cooling is used instead of oil cooling. The structure after tempering is tempered troostite with a hardness of 40-46HRC. It has good elasticity and sufficient strength and toughness. In addition, if the tempering time is too short, the uniform structure and performance cannot be obtained, and the performance is not improved if the time is too long. Therefore, a process test should be carried out to determine a reasonable time.
